Handle buffers created with MapRead

[greggman]

Buffer usages (BufferUsage::(MapRead|CopySrc|CopyDst)) is invalid. If a buffer usage contains BufferUsage::MapRead the only other allowed usage is BufferUsage::CopyDst.

[Kangz]

I'm not sure what this issue is about, but in general I don't think that we should support replays of applications with validation errors, it will be difficult to shield against error propagation in readbacks etc in that case.

[greggman]

The application doesn't have validation errors. The debugger is adding flags

[Kangz]

Ah ok, so this issue is about mappable buffers not being special cased in the capture code, so flags are added and there's a validation error.